1. Field of the invention
This invention relates to a paint spraying apparatus particularly suitable for use, for example, in coating paint on vehicle bodies or the like.
2. Discussion of the Background
Generally, paint spraying apparatus which are in wide use for coating a paint on vehicle bodies or the like are largely constituted by a color changing valve assembly unit which is connected to a multi-color paint source for supplying paint of specified colors selectively from the multi-color paint source, and a paint applicator unit which is connected to the color changing valve assembly unit to spray a paint toward an object to be coated. In a paint spraying apparatus of this sort, in many cases, the paint applicator unit is either mounted on a coating robot or on a reciprocator which is located or installed in a plate remote from the color changing valve assembly, and connected with the latter through a number of paint feed pipes.
In a paint spraying apparatus of this class, for the purpose of enhancing the efficiency of paint deposition on a coating object, a high voltage is applied to the paint applicator unit by the use of a high voltage generator, letting charged paint particles sprayed by the paint applicator unit fly along an electrostatic field which is formed toward the coating object which is retained at the earth potential.
Further, the paint applicator units which are generally used on the paint spraying apparatus of this sort can be classified into a rotary atomizing head type paint applicator unit having a rotary atomizing head which is put in high speed rotation by an air motor to spray a paint supplied from a color changing valve assembly, and a spray gun type paint applicator unit adapted to spray a paint supplied from a color changing valve assembly, along with compressed air or under pressurized air.
Further, the paint applicator units which are in use on the paint spraying apparatus of this sort can also be classified into a straight type paint applicator unit having a housing which is rectilinearly straight in shape for spraying a paint in a straightforward direction and which is suitable for coating surfaces with relatively simple shapes or contours like exterior surfaces of vehicle bodies, and a bend type coating unit having a housing which is angularly bent and suitable for spraying a paint in an angular direction off its axis for coating surfaces with relatively complicate shapes or contours like interior surfaces of vehicle bodies.
In this connection, Japanese Patent Laid-Open No.H6-134354 discloses an electrostatic paint coating apparatus which is generally constituted by a synthetic resin casing, an air motor assembled into the resin casing, a rotary atomizing head mounted at the fore distal end of a rotational shaft of the air motor, and a paint nozzle assembly which can supply paint of different colors to the rotary atomizing head. In this instance, the paint nozzle assembly is provided with a plurality of paint nozzles exclusively for the respective colors which are available, without necessitating the wasting residual paint each time the paint color is changed.
Also disclosed in Japanese Patent Laid-Open No.H7-213957 is an electrostatic paint coating apparatus consisting of a single machine body including an air motor assembled into the machine body, a rotary atomizing head mounted at the fore distal end of a rotational shaft of the air motor, a paint feed pipe for supplying paint to the rotary atomizing head, and a color changing valve mechanism assembled into the machine body in a position on the rear side of the air motor and having a plurality number of paint feed valves in association with a washing valve. In this case, a paint feed passage which connects the paint feed valves with the rotary atomizing head is shortened to minimize the amount of paint which has to be wasted at the time of changing the paint color.
In this regard, of the prior art paint spraying apparatus mentioned above, the paint spraying apparatus which has a color changing valve assembly unit located in a distant place away from a paint applicator unit has an inherent drawback that a large amount of residues of previous color, which remains between the color changing valve assembly unit and the paint applicator unit, has to be wasted and replaced by a paint of new color each time when changing the paint color.
On the other hand, in the case of Japanese Patent Laid-Open No.H6-134354 employing a number of paint nozzles exclusively for the respective colors to be used, there is a problem that a high voltage is always applied not only to a currently selected paint but also to all the paint portions which are standing by within the respective paint nozzles allotted to different colors. As a consequence, under the influence of high voltage, pigments or other components in standing-by portions of paint are caused to separate from solvent and to deposit or sediment within the respective paint nozzles. In some cases, the paint deposits of this sort result in clogging of the paint nozzles. Besides, flakes of paint deposits peeled off and released from walls of paint feed passages could be a cause of serious defects or flaws if they deposit on coated surfaces of products.
In addition, in case it is a metallic paint that is standing by in a paint nozzle, there will arise another problem that coatings are often flawed by the "bridge phenomenon", i.e., electrical shortcutting which occurs to metallic pigments on application of a high voltage.
On the other hand, in case a plurality of paint valves and washing valves are incorporated into a single machine body which is applied with a high voltage as in Japanese Patent Laid-Open No.H7-213957, similarly high voltage is applied not only to a paint of a selected color but also to all the paint portions which are standing by in the respective paint valves. Therefore, similarly to above-mentioned Japanese Patent Laid-Open No.H6-134354, the paint coating apparatus of Japanese Patent Laid-Open No.H7-213957 suffers from the problem of separation and deposition of paint pigments within the respective paint valves under the influence of high voltage.
Further, with paint spraying apparatus which are employed for coating vehicle bodies, it is often the case that a paint applicator unit of either a rotary atomizing head type or a spray gun type is selectively used depending upon conditions of coating operations. For example, a paint applicator unit with a straight housing is generally used when coating exterior surfaces of vehicle bodies, while a paint applicator unit with a bent housing is used for coating interior surfaces of vehicle bodies.
Therefore, it is usually necessary to provide different types of spraying coating units to cope with different conditions of coating operations, and as a result, there is the necessity for providing a large number of component parts which are designed exclusively for the respective paint applicator units of different types, despite degradations in production efficiency and increases in production cost. On the part of users, it has been necessitated to keep a large inventory of spare parts.
In this connection, in the case of an electrostatic paint coating apparatus which has a color changing valve mechanism integrally built into a single machine body as in Japanese Patent Laid-Open No.H7-213957, it is necessary to provide a color changing valve mechanism in each one of various types of paint applicator units, including a plurality of paint feed valves, washing valves and many other component parts. As a result, the construction of each paint applicator unit necessarily becomes complicated, involving an increased number of component parts which will be reflected by degradations in efficiency of assembling work and increases in production cost.